When pedaling a bicycle or other pedal drive vehicle, a large amount of pedaling force may be transferred from the crank to the drive wheel(s) when the cranks are close to a horizontal position. However, it may be desirable to have substantially constant force during the revolution of the pedals, even torque at all times. The human body is able to put more force on the pedals when the crank is closer to horizontal, (i.e., pushing down on the pedal for the crank moving downward and pulling up on the opposite pedal for the crank moving upward). When the cranks are closer to a vertical position, the feet produce forward horizontal force on one pedal and backward horizontal force on the opposite pedal. The human body typically does not produce the same force in these “dead spots” of the revolution cycle, due to a person's biomechanics. In other words, the torque curve of a crank assembly through a full crank stroke may have sharp torque spikes and valleys. As a result, drive wheel traction may be reduced when the torque spikes are transferred to the drive wheel via a drivetrain. Moreover, a user's muscles may become fatigued more quickly when they exert uneven pedal forces during the crank stroke, which may lead to exhaustion, decreased power output and therefore vehicle speed.
Several attempts have been made to remedy the “dead spot” problem. For instance, elliptical or oval chain rings have been developed in an effort to decrease the magnitude of the peaks and valleys in a crank's torque curve. For instance, elliptical or oval chain rings are available on the market under the names O'Symetric, Biopace chain wheel, Rotor Q-ring and Ridea® Power Ring. Additionally, U.S. Pat. No. 5,636,554 discloses a variable length crank arm where a secondary crank arm is biased towards a retracted position. Furthermore, U.S. Pat. No. 5,611,744 discloses a variable shape chain ring which deforms in shape during a crank stroke in an effort to even out the crank's torque curve. Other solutions include, a spring-loaded bicycle cranks such as the cranks disclosed in U.S. Pat. No. 5,279,524 and U.S. Pat. No. 6,161,452. The spring-loaded bicycle cranks include a spring coupled to a crank arm and a chain ring. Other types of spring-loaded cranks may include two springs suspended between the frame and the chain crank mechanism.
Yet another attempt to remedy the problem of “dead spots” includes a Rotor Crank in which the cranks are not fixed at 180°, but are variable during a pedal cycle. The cranks are synchronized through a set of cams that change the angle between the crank arms producing a variation in transmission ratio during each cycle. The system relies on an eccentric bearing on the bottom bracket and the right crank movement is then controlled by the two cams accelerating the crank through the “dead spot” at the top and bottom of the pedal stroke and prolonging the power stroke phase. By the time one crank is at the bottom of the stroke and in the six o'clock position, the opposite crank has been accelerated through to the one o'clock position and has already started the power stroke phase.
Each of the designs listed above have multiple drawbacks. For instance, each of the designs may be costly. As a result, none of the designs have been widely adopted. Additionally, drawbacks of oval chain rings include making it more difficult to switch gears in the vehicle and in many oval type crank assemblies only the outer chain ring is oval, limiting the amount of gear ratios where the “dead spots” can be attenuated. Additionally, the use of an oval chain ring leads to slack and rattling of the chain. Still further, the degree of ellipticity of the chain ring cannot be changed and is therefore typically not suited for every riding condition. For example, on a 10° or higher riding surface slope an oval shape with the longer diameter perpendicular to the crank may actually be disadvantageous, while on flat terrain it is beneficial. This is because the dead spot reduction remains perpendicular to the riding surface (i.e., at 10° off vertical) rather than moving to the vertical position which is much more beneficial. Furthermore, the variable length crank arm disclosed in U.S. Pat. No. 5,636,554 stores energy in the arm when a distance between an aperture for a pedal and a rotational axis of the crank arm is increased. This configuration may lead to an undesirable path of the pedal, which may cause undue stress and strain on a rider's joints, ligaments, and muscles. Consequently, the likelihood of a rider developing injuries from the variable length crank arm disclosed in U.S. Pat. No. 5,636,554 is increased.
In the Rotor crank the chain wheel and the crank also rotate independently. Compatibility with conventional frames has been resolved for the rotor crank, but its weight is significantly increased and, like the oval chain ring, only one setting exists, which may not fit all riding conditions (e.g., uphill, time trial, sudden accelerations). With regard to spring-loaded type bicycle cranks they may necessitate a total redesign of the crank set, due to the fact that the crank and chain wheel rotate independently. Consequently, a specialized frame may be necessitated for use with the spring-loaded crank due to the configuration of the chain wheel and crank, making the crank set incompatible with many types of frames. As a result, none of the aforementioned designs has been widely adopted.